1. Technical Field
This invention relates to a method and apparatus for examining a subject.
2. Prior Arts
In general, apparatuses for examining portions of a human body for diagnostic purposes suffer more or less from artifacts produced in an obtained tomogram or lowering of resolution due to movements of the subject or movements of internal organs of the subject. To minimize such artifacts or lowering of resolution, it is essential to shorten a measuring time. To this end, there has been proposed a measuring apparatus comprising an X-ray tube for generating a fan beam type X-ray having a given spreading angle and a plurality of X-ray detectors and adapted to make measurement while rotating the X-ray tube and the detectors group around the subject to be examined, keeping a given positional relation therebetween. This type of apparatus can greatly reduce a scanning time as compared with a conventional apparatus which makes a measurement only by traverse and rotation movement.
However, if the detectors of such an apparatus have individual differences in X-ray detecting performance (herein after referred to as an "X-ray detecting characteristic"), there are again caused artifacts or lowering of resolution in an obtained image. In fact, it is reported by papers etc. that a noise due to such differences between individual characteristics of the detectors appears in the form of a ring in an obtained computed tomogram (CT) image. To prevent these phenomena, it is essential that not only each detector have general characteristics as a radiation detector but also it have highly accurate, uniform and stable energy response or characteristics such as sensitivity, offset, linearity, directional characteristic and ray-quality characteristic.
However, in fact, it is quite difficult to prepare such ideal detectors. By this reason, efforts have been made to effectively improve characteristics of the detectors by improving a measuring method. As to uniformalization of sensitivity of the detectors, there has been proposed a method wherein X-rays are preliminarily irradiated before a subject is placed in the machine, the sensitivities of the respective detectors are stored in a memory and measured values obtained upon actual measurement of the subject are divided by the stored sensitivities to make calibration. There has also been proposed a method for eliminating offset errors. This method comprises a step of applying pulsed X-ray just before measurement, a step for storing, into a memory, output values, i.e. offset values of the respective detectors obtained when the X-ray is not applied, namely, during intervals of the pulses, and a step of cancelling outputs of the detectors obtained during the measurement with the stored data of the offset values. According to this method, not only the offset errors can be eliminated but also an influence by variation with time can be reduced.
However, energy response such as linearity, directional characteristic, and ray-quality characteristic inherent of the respective detectors cannot easily be corrected. For example, in a detector comprised of a Xenon gas chamber, a plurality of electrodes are arranged in parallel with each other within a casing, and it has been known that, when the detectors of this type are arranged so as to form a multi-channel detectors group, there are problems such as electric fields of respective electrode ends of the detectors are not uniform, the respective electrodes have delicate differences in their formation and the respective detectors have different insulation resistances. Therefore, it is quite impossible to provide highly uniformalized characteristics of the detectors, such as linearities, directional characteristic, and ray-quality characteristic. The differences in characteristics between individual detectors can be regarded as errors. To correct such errors of the respective detectors, there has been proposed the following method: a plurality of phantoms having uniform beam passage lengths and uniform absorptivities are placed by turns in the course of an X-ray beams (a measuring path); the X-ray is irradiated onto the respective phantoms, a penetrated X-ray obtained through the phantoms is detected by a multi-channel type detectors group; error values due to non-uniformity in output levels of the detectors are stored so as to correspond to the respective detectors and the respective output levels; the error values preliminarily measured are used for calibration in measurement of a subject by applying the error values to the outputs of the detectors obtained in the subject measurement according to the levels thereof. In accordance with this method, as the absorption of the uniform phantom increases, the ray quality of the X-ray incident upon the detectors becomes harder. Therefore, if the subject and an X-ray absorption spectrum of the uniform phantom are made of like materials, the non-uniformity in linearity and ray-quality characteristic can be highly improved. However, this method cannot correct errors in directional characteristic of the detectors. More specifically, when fan beam type X-rays produced from an X-ray tube are irradiated onto the subject, there occurs scattering of the X-ray within the subject so that not only a main X-ray but a scattering X-ray produced in the subject is detected in the respective detectors. If there is no difference or variability in directional characteristics among the individual detectors, the scattering X-ray only imparts spacially biases to the respective detectors in a low frequency range. On the other hand, if there are differences or variabilities in directional characteristics, bias components imparted to the respective detectors produce errors in outputs of the detectors related to the contour of the subject when the subject is actually made to undergo a measurement. Therefore, the errors due to the scattering X-ray cannot be eliminated by such correcting means as mentioned above. A method for correcting errors due to such a scattering X-ray and ray quality is described herebelow. A phantom is first prepared which has an X-ray absorbing characteristic similar to that of a subject and a contour similar to that of the subject and is made of a material uniform at least within a region to be observed. Images of the phantoms are obtained according to the size and contour of subjects, and subtraction is carried out between the image of the subject and the phantom image. This method performs substracting operation between the data corresponding to the absorption value obtained by measuring the phantom and the data corresponding to the absorption value obtained by measuring the subject so that errors due to the ray-quality characteristic and scattering X-ray can be equivalently reduced. In this method, however, fitness of a material, size and configuration of the phantom to the subject is an important factor of reducing errors. Therefore, in actual application of the method, it is required to prepare various phantoms adapted for subjects to be measured and to sample various error data thereof. In addition, an obtained image is effective only for a portion of the phantom where an absorptivity distribution is uniform. A portion out of the range where the absorptivity distribution is uniform produces artifacts. As a result, an effective region becomes narrow in the measurement and it is required to set the subject in the narrowed effective region for every measurement.